SITE INFORMATION AND CORRESPONDENCE

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Mr. Gerald A. Stauffer - 2 - 15 February 2006 Valero Logistic Operations,LP appropriate for natural attenuation of petroleum hydrocarbons. Two new monitoring wells would be installed at the downgradient edge of the plume to assist in MNA monitoring. Central Valley Regional Board staff does not concur with the Cleanup Plan recommendation for NINA as the preferred remedial alternative for this site. There are several items that do not adequately demonstrate that MNA is a viable remedial alternative. The information below is provided to explain the issues regarding the recommendation for MNA. 1. Plume definition: The eastern extent of the Total Petroleum Hydrocarbons as gasoline (TPHg),Total Petroleum Hydrocarbons as diesel (TPHd), benzene, and methyl tertiary butyl ether(MTBE)plumes is not well defined in A-zone and B-zone groundwater. For example in the 'third quarter 2005, TPHg was present up to 7,100 micrograms per liter (µg2) in A-zone wells along Stork Road and two wells to the east where TPHg was reported as non-detect are located 400-to 500 feet away. A similar situation exists for TPHd and benzene in A-zone groundwater, though high concentrations of these compounds exist at fewer wells along the east side of the site. MTBE in A-zone groundwater was non-detect at wells PS/P-11, PS/P-12, and PS/P-13 in the third quarter 2005 although at elevated reporting limits, but was reported at a concentration of 1,400 gg/L at well PS/MW-15. MTBE was reported at 39 µg2 at well PS/WC-3S approximately 500 feet east and downgradient of well PS/MW-15. Valero concludes that MTBE reported at well PS/WC-3S is not related to petroleum releases from the site because the chemical signature of petroleum releases prior to 2002 at the facility had a low fraction of MTBE. TPHg, TPHd, benzene, and MTBE have historically been reported at B-zone well PS/WC-3M. In the third quarter 2005, MTBE was reported at 240 µg2 at well PS/WC- 3M. MTBE was not analyzed at wells OW-713 and OW-813 downgradient of well PS/WC-3M, but was reported at a concentration of 11 µg/L at well PS/WC-4M southwest of well PS/WC-3M. Valero states that MTBE reported at well PS/WC-3M may be from the northern leasehold, ST Services and/or the north adjacent SFPP facility, or a source near the well. Although TPHg, TPHd, benzene, and MTBE concentrations at well PS/WC-3M have decreased over time, there is no evidence to show that the contamination is not contiguous with the onsite plume as depicted in Cleanup Plan Figures 8, 9, and 10. Valero should provide better characterization of the extent of the plumes east of the site, such as installing and sampling wells between high concentration and non-detect locations, along with a demonstration of stable to decreasing concentration trends at the fringe of the plume to support the recommendation for NINA. 2. Plume stability: Figures 8, 9, and 10 provided in the Cleanup Plan show the estimated extent of A-zone groundwater TPHg,benzene, and MTBE plumes, respectively, in 1995, 2001, 2003, and 2005. The plume limits are apparently based on decreasing contaminant concentrations at wells interior to the depicted limits. As stated above, Valero should provide more characterization data between high concentration and non-detect wells to demonstrate stability or decreasing plume size.